LED with patterned surface features based on emission field patterns

ABSTRACT

The escape surface of a light emitting element includes features that include sloped surfaces that have angles of inclination that are based on the direction of peak light output from the light emitting element. If the light output exhibits a number of lobes at different directions, the sloped surfaces may have a corresponding number of different angles of inclination. To minimize the re-injection of light into adjacent features, adjacent features may be positioned to avoid having surfaces that directly face each other. The features may be shaped or positioned to provide a pseudo-random distribution of inclined surfaces across the escape surface, and multiple roughening processes may be used.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/900,143 filed on Dec. 18, 2015, and titled “LED WITHPATTERNED SURFACE FEATURES BASED ON EMISSION FIELD PATTERNS,” which is a§ 371 application of International Application No. PCT/IB2014/062150filed on Jun. 12, 2014, which claims the benefit of U.S. ProvisionalPatent Application No. 61/836,775 filed on Jun. 19, 2013. U.S. patentapplication Ser. No. 14/900,143, International Application No.PCT/IB2014/062150, and U.S. Provisional Patent Application No.61/836,775 are incorporated herein.

FIELD OF THE INVENTION

This invention relates to the field of optics, and in particular to thecreation of patterns on a light-extraction surface, such as a surface ofa light-emitting device.

BACKGROUND OF THE INVENTION

A conventional semiconductor light emitting element includes a doubleheterostructure that includes a light emitting (‘active’) layer that issandwiched between an N-type clad layer and a P-type clad layer. Whencharge-carriers (electrons and holes) flow into the active layer, thesecharge-carriers may meet. When an electron meets a hole, it falls into alower energy level, and releases energy in the form of a photon. Thecreated photon may travel in any direction, and commercially availablelight emitting elements typically include reflective surfaces thatredirect light so as to exit an intended escape surface of the lightemitting element. However, the light may strike the escape surface atvirtually any angle, and a substantial portion of the light may strikethe surface at an angle that exceeds a critical angle of the interfacebetween the materials on either side of the surface and be totallyinternally reflected (TIR).

The critical angle is determined by the indices of refraction n1 and n2of the material at an interface between the materials, and is equal to:arcsin(n2/n1),  (Equation 1)for light traveling from the medium having an index of refraction n1into a medium having an lower index of refraction of n2. Light thatstrikes the surface at greater than the critical angle will be totallyinternally reflected, and will not escape through the surface. The term“escape zone”, or “escape cone” is used to define the range of angleswithin which light will escape through the surface. The escape cone atany point on the surface is a cone with an apex at the surface whosecross-section subtends an angle of twice the critical angle about anormal to the surface. The escape zone is the composite of the escapecones of all points on the surface.

Although escape zones are defined by solid angles, this disclosure ispresented using a two-dimension model, for ease of presentation andunderstanding. One of skill in the art will recognize that theconclusions drawn from the following analysis of two dimensional opticalmodels are applicable to a more complex analysis using athree-dimensional model.

It has been determined that roughening the escape surface allows morelight to escape through the surface, compared to a smooth surface. Whenlight is totally internally reflected from the smooth escape surface, itwill travel back toward the interior of the light emitting element, bereflected by the reflective surfaces, and redirected back toward theescape surface. In most cases, this process is repeated until thereflected light is fully absorbed inside the LED. Conversely, because aroughened surface will have portions of its surface at varying anglesrelative to the surface of the active area, some of the light that wouldhave been outside the escape zone of a smooth escape surface will bewithin the escape zone of a sloped surface of the roughened surface andwill exit the roughened escape surface; additionally, some of the lightthat may be reflected from the roughened escape surfaces may beredirected in the desired direction (e.g. orthogonal to the activelayer), so that on the next bounce, the likelihood of exiting the escapesurface is increased.

The escape surface of the light emitting element may be roughened usingany of a variety of techniques, some of which create a randomlyroughened surface, and some of which create a surface with a particularpattern of grooves, crevices, structures, and the like. In “RecentProgress of GaN Based High Power LED” (14^(th) Optoelectronics andCommunication Conference, 2009), Hao-chung Kuo discloses a combinationof roughening techniques wherein the escape surface is first patterned,then subjected to a random roughening process, creating an escapesurface having a roughened pattern.

Although roughening the escape surface improves the light extractionefficiency, some of this efficiency is lost when light that exits afeature on the roughened escape surface strikes an adjacent feature andis ‘re-injected’ into the light emitting element. Additionally, in arandom roughening process, control of the shape and density of thecreated features is somewhat limited, and hence, the likelihood of lightexiting the surface, and the likelihood of light being re-injected intothe surface, is difficult to control or predict.

SUMMARY OF THE INVENTION

It would be advantageous to further improve the light extractionefficiency of an escape surface of a light emitting element. It wouldalso be advantageous to use a patterned surface that is featured toincrease the light extraction efficiency based on the particularcharacteristics of the light source of the light emitting element.

To better address one or more of these concerns, in an embodiment ofthis invention, the escape surface of a light emitting element includesa plurality of features, each feature having a plurality of surfacesthat have angles of inclination that are based on the direction(s) ofpeak light output from the light source. If the light output exhibits anumber of lobes at different directions, the plurality of surfaces mayhave a corresponding number of different angles of inclination. Tominimize the re-injection of light into adjacent features, adjacentfeatures may be positioned to avoid having surfaces that directly faceeach other.

In particular, a light emitting element emits light with a particularemission field pattern, and includes an escape surface from which thelight is emitted. To improve light extraction efficiency, the escapesurface includes surface features that include sloped surfaces withslopes that are dependent upon the particular angular emission fieldpattern of the light emitting element. The surface features may include,for example, conic or pyramid features having slopes that maximize theamount of light that is able to escape the escape surface. The surfacefeatures may be formed by etching, milling, or laser slicing.

The interface between the sloped surfaces and an exterior mediumexhibits a critical angle that defines an escape zone outside of whichlight is totally or mostly internally reflected, and the slopes aredetermined to maximize an amount of light that strikes the slopedsurfaces within the escape zone.

The far field angular emission field pattern may include a plurality oflobes, and the slopes of the features on the escape surface aredetermined based on the polar orientation of these lobes. In someembodiments, different surface features may be created to provide slopedsurfaces with different slopes, the different slopes being dependentupon the different lobes that may be present in the particular emissionfield pattern.

To minimize the likelihood of emitted light re-entering the lightemitting element, the surface features are arranged on the surface suchthat none of the sloped surfaces of a surface feature directly face anysloped surfaces of a neighboring surface feature. For example, in someembodiments, each sloped surface of a surface feature faces an edge of aneighboring surface feature.

In like manner, if a particular process provides for the efficientcreation of particular surface features, or if particular surfacefeatures are known to provide high extraction efficiency and lowre-insertion likelihood, the elements of the light emitting device maybe arranged to produce an angular emission field pattern that realizesoptimal or near-optimal light extraction efficiency through theseparticular surface features.

In some embodiments, the surface features of the roughened surface andthe angular emission field pattern of the light emitting element aredesigned in unison to provide for a light emitting element with highlight extraction efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in further detail, and by way of example,with reference to the accompanying drawings wherein:

FIG. 1 illustrates an example set of light emission field patterns forvarious distances between the light emitting layer and a reflector.

FIGS. 2A-2B illustrate example polar plots of light emission fields.

FIGS. 3A-3C illustrate example surface pattern profiles based on peaksin the light emission fields.

FIGS. 4A-4B illustrate example surface features formed as conicfeatures.

FIGS. 5A-5B illustrate example surface features formed as pyramidfeatures.

FIGS. 6A-6C illustrate example light paths, including light paths thatre-enter the light emitting element.

FIGS. 7-10 illustrate example arrangements of surface features thatreduce the likelihood of light re-entering the light emitting element.

FIG. 11 illustrates an example escape surface comprising a pseudo randomarrangement of surfaces.

FIG. 12 illustrates an example escape surface comprising a circulararrangement of features.

FIG. 13 illustrates an example light emitting surface and slopedfeature.

Throughout the drawings, the same reference numerals indicate similar orcorresponding features or functions. The drawings are included forillustrative purposes and are not intended to limit the scope of theinvention.

DETAILED DESCRIPTION

In the following description, for purposes of explanation rather thanlimitation, specific details are set forth such as the particulararchitecture, interfaces, techniques, etc., in order to provide athorough understanding of the concepts of the invention. However, itwill be apparent to those skilled in the art that the present inventionmay be practiced in other embodiments, which depart from these specificdetails. In like manner, the text of this description is directed to theexample embodiments as illustrated in the Figures, and is not intendedto limit the claimed invention beyond the limits expressly included inthe claims. For purposes of simplicity and clarity, detaileddescriptions of well-known devices, circuits, and methods are omitted soas not to obscure the description of the present invention withunnecessary detail.

As noted above, a conventional light emitting element may include areflective surface that reflects light from the active layer toward theescape surface. The photon radiation from an active layer near a silvermirror exhibits the characteristics of an electric dipole in thevicinity of a metal plane. The resultant far field radiation patternwill be dependent upon the distance between the active layer and themirror relative to the wavelength of the emitted light, as well as therefractive index of the material through which the light travels. Givena distance D between the active layer and the mirror, a relativedistance d may be defined as:d=η*D/λ,  (equation 2)where:

-   η is the index of refraction of the material between the active    layer and the mirror; and-   λ is the wavelength of the emitted light.

FIG. 1 illustrates an example set of normalized far field emissionpatterns 110-200 for different values of the relative distance d betweenthe active layer and the mirror. In this example, the material betweenthe active layer and the mirror is GaN, and the mirror is a silverreflector. The vertical axis represents the normalized emissionmagnitude, and the horizontal axis represents the angle relative to anormal to the light emitting surface.

As can be seen, when the mirror is very close to the active layer(d=0.1), the emission pattern is not significantly different from aLambertian pattern, with a peak emission levels at angles at or nearnormal to the surface, and progressively lower emission levels as theangle increases away from normal. As the relative distance increases,the travel through the increased distance D allows for interactions thatcreate increasingly complex patterns.

As the relative distance increases from 0.1 to 0.4, less light isemitted in the normal direction. As can be seen, in the pattern 140produced when the relative distance is 0.4 (D=0.4*λ/η), there is verylittle emission in directions at or near normal, and a large amount ofemission 146 in directions at or near 60 degrees off normal. As therelative distance increases further, toward a relative distance of 0.6(D=0.6*λ/η), multiple peaks (152, 156), (162, 166) are produced as theamount of light emitted in the normal direction increases. As therelative distance is further increased, the multiple peaks are offsetfurther from normal, and additional peaks are formed.

Note that different patterns may be produced under differentcircumstances. For example, the patterns illustrated in FIG. 1 arepatterns that may be produced assuming near-perfect reflection from themirror; in reality, mirror surfaces are rarely perfect reflectors.Additionally, various other parameters may affect the emission fieldpattern. FIGS. 2A and 2B illustrate two different emission fieldpatterns for two models of a light emitting device with differentparameters taken into account. FIG. 2A illustrates a predicted emissionfield pattern using a ‘simple’ model, corresponding, for example, to thepattern 160 of FIG. 1, while FIG. 2B illustrates a more complex model ofthe same light emitting device.

FIGS. 2A and 2B provide a ‘polar’ representation of the far fieldemission field pattern, wherein the loci of points on the semi-circlerepresents the angles relative to the surface of the light emittinglayer, and the distance from the origin represents the amplitude of theemissions at the particular angle. In such a representation, emissionsabout a peak emission amplitude appear as “lobes”. In FIG. 2A, threelobes 210, 220, 221 are indicated, although in three-dimensional space,lobes 220 and 221 are cross sections of the same ‘solid angle lobe’having an angular orientation of about 70 degrees. In FIG. 2B, fourlobes 230, 240, 231, 241 are indicated, corresponding to two solid anglelobes at peak emission angles of about 30 degrees and 70 degrees. Forease of reference, the term ‘lobe’ is used hereinafter to indicate‘emissions about a peak emission angle’.

The examples of FIGS. 1 and 2A-2B are merely intended to illustratethat, depending on the fabrication technique and parameters,conventional light emitting devices may exhibit non-Lambertian emissionpatterns. As will be evident below, this invention is not limited to theuse of the dipole-mirror effect to introduce any particular emissionpattern; rather, it will be apparent to one of skill in the art thatthis invention is not dependent upon any particular technique used tocreate the particular emission field pattern.

Although the example of FIG. 1 illustrates that the far field emissionfield pattern may be controlled by controlling the distance between thereflector and the active layer, so that off-normal peaks can be avoidedif desired, in many instances, the fabrication process is constrained byfactors that do not allow for control of the emission field pattern, perse. Any particular fabrication technique, however, will generallyproduce a corresponding particular emission field pattern, and testingmay be performed to determine the particular emission field pattern (orset of lobes) of devices produced by the given technique.

In an embodiment of this invention, the emission field pattern of agiven light emitting device, or class of light emitting devices, isdetermined, and the escape surface of the light emitting device ispatterned, based on the determined emission field pattern, to reduce thetotal internal reflection (TIR) at the surface caused by light strikingthe surface at angles greater than the critical angle off normal to theescape surface.

In other embodiments of this invention, the angular light extractionefficiency of a given set of escape surface features is determined, andthe emission field pattern of a light emitting element is designed basedon the determined angular light extraction efficiency. In someembodiments, the escape surface features and the emission field patternare designed in unison to achieve a high light extraction efficiency.For ease of reference and presentation, the following disclosure ispresented using the paradigm of designing the escape surface features tooptimize the light extraction efficiency for a given emission fieldpattern. One of skill in the art will recognize that the principlespresented using this paradigm are applicable to the otherabove-mentioned design sequences.

Also for ease of reference and presentation, the following examples arepresented using the aforementioned two-dimensional model of the emissionfield patterns, and using an idealized model of total internalreflection wherein all light within the escape zone exits the surface,and all light outside the escape zone is totally internally reflected.

FIG. 3A illustrates an example surface profile with features 310 each ofwhich includes sloped surfaces 312, 314. In this profile, a lightemitting region 302 is sandwiched between n-type and p-type regions 301,303. The sloped surfaces 312,314 are formed in region 303. A mirror 316is formed at a distance D below light emitting region 302. One of skillin the art will recognize that each of these regions 301, 302, 303 mayinclude multiple layers of materials, including contacts and the like(not illustrated).

In this example, the sloped surfaces 312, 314 are at a 45 degree anglerelative to the surface 320 of the light emitting region 302. Light thatis emitted from the surface 320 at 45 degrees will strike one of thesloped surfaces 312, 314 at an angle that is normal to the slopedsurface, and will exit the surface. Of particular note, the criticalangle at the sloped surfaces 312, 314 will be relative to the normal ofthese surfaces 312, 314, which is at a 45 degree angle from the normalto the light emitting surface 320.

Light that is emitted at 30 degrees off normal from the light emittingsurface 320 will strike one of the sloped surfaces 312, 314 at an angleof 15 degrees, as will light that is emitted at 60 degrees. If thecritical angle is at least 15 degrees, light emitted at angles between30 and 60 degrees that strikes the surfaces 312, 314 will exit thesesurfaces. Contrarily, light that is emitted at 0 degrees (normal to thelight emitting layer) will strike the surfaces 312, 314 at an angle of45 degrees, and may be totally internally reflected if the criticalangle is less than 45 degrees. In embodiments of this invention, theslope of the sloped surfaces 312, 314 may be determined so as tominimize the amount of light that is totally internally reflected, basedon the emission pattern of the light emitted within the light emittingdevice.

If a light emitting device is determined to exhibit an emission patternsimilar to the pattern 140 in FIG. 1, for example, a significant amountof light will be emitted at angles at or near 60 degrees off normal(i.e. a lobe is present at about 60 degrees, and −60 degrees, in the twodimensional ‘cross section’ of the three-dimensional emission pattern;in three dimensions, this lobe is revolved about an axis orthogonal tothe light emitting surface). If the escape surface is parallel to thesurface of the light emitting layer, and the critical angle of theescape surface is, for example, 40 degrees, a substantial portion of theemitted light will be outside of the escape zone of the escape surface,and will be totally internally reflected when it strikes the escapesurface.

To increase the amount of light that will exit the escape surface, thesurface may be patterned to create surface features that provide escapezones that encompass a significant portion of the emitted light. Forexample, if as in the above example, a lobe is present at 60 degrees offnormal to the light emitting surface, features may be created on thesurface that present surface areas that are sloped 60 degrees relativeto the plane of the light emitting surface. With such a sloped surface,the center of the escape zone will be oriented in the same direction asthe peak of the lobe at 60 degrees, and light that strikes this slopedsurface within 60 degrees+/−the critical angle will be able to escapethrough the sloped surface. Since a significant amount of the emittedlight will be at an angle of 60 degrees+/−the width of the lobe, asignificant amount of the emitted light within the lobe that strikes thesloped surface will escape through the sloped surface.

If, as in the above example, the critical angle is 40 degrees, the lightthat is emitted above 20 degrees and strikes the surface feature that issloped at 60 degrees will exit the escape surface. As illustrated inFIG. 1, if a device exhibits a pattern similar to pattern 140, almostall of the emitted light is emitted at angles above 20 degrees.Accordingly, almost all of the emitted light that strikes the surfacefeature is able to exit the escape surface.

One of skill in the art will recognize that the emitted light is emittedin a direction relative to three dimensions, and the direction of anemission may be referenced by angles with respect to two orthogonal axesin this three-dimensional system, as illustrated in FIG. 13. In thisexample, emitted light 30 is at an angle 31 relative to axis 10, whichis normal to the light emitting surface 50, consistent with the anglesreferenced in FIGS. 1 and 2. The other angle 32 is an angle relative toa reference line (axis) 20 on the light emitting surface 50. In likemanner, the orientation of a sloped surface 60 may be referenced withrespect to these same two axes. That is, in addition to being slopedwith regard to a normal 10 to the light emitting surface 50, the surface60 will also ‘face’ the light emitting surface from a given directionrelative to the axis 20 on the light emitting surface.

With regard to the escape zone of the sloped surface 60, the angle ofintersection of emitted light 30 upon the sloped surface 60 will berelative to a normal axis 65 that is orthogonal to the sloped surface 60and will be a composite of the emission angles and slope angles withrespect to the axes of the light emitting surface. Additionally, if thesloped surface is curved, the normal 65 will vary across the curvedsurface. Light that strikes a point on the sloped surface 60 will escapeonly if this composite angle of intersection with respect to the normal65 at the point is within the escape zone of the sloped surface 60.

Simulation and analysis systems are commonly available for determiningwhether light from a given 3D direction will exit a surface oriented atanother direction based on the critical angle relative to a normal tothe surface. However, as noted above, for ease of explanation andunderstanding, the principles of this invention are presented using theaforementioned two-dimensional orientation of the surface featuresrelative to the plane of the emitting surface. One of skill in the artwill recognize that a three-dimensional analysis will follow along thesame principles; except that the angles of intersection will be based ona three dimensional model such as illustrated in FIG. 13.

Returning to the two-dimensional analysis, it should be noted that thesloped surface need not be strictly perpendicular to the angle of lobes.In the example above, with a critical angle of 40 degrees, and a peakemission angle of 60 degrees, a surface that is sloped at 50 degreeswill allow all of the light that is emitted at angles above 10 degreesand strikes the surface to exit the escape surface; a surface that issloped at 45 degrees will allow the light that is emitted between 5 and85 degrees (45+/−the critical angle) to exit. The particular slope ofthe surface is preferably selected to optimize the amount of emittedlight that will exit that surface, based on the amount of light that isincluded within the escape zone of the sloped surface from the emissionfield pattern of the light emitting device.

If, for example, the light emitting device exhibits a pattern similar topattern 130 of FIG. 1, wherein the peak/lobe occurs at about 55 degrees,it is noted that substantially more light is emitted at angles less than55 degrees than at angles greater than 55 degrees. Accordingly, theescape zone of the surface may be biased toward the angles of greateremissions, such as to 45 degrees, even though the peak/lobe occurs atabout 55 degrees. In this manner, although some of the light at thehigher angles of the lobe may not be included in the escape zone of thislesser sloped surface, the potentially greater amount of light that isemitted at the lower angles of the lobe may be expected to be includedin the escape zone and will consequently exit the escape surface.

Using pattern 130 and a critical angle of 40 degrees again as theexample, the amount of light emitted between 70 and 80 degrees appearsto be less than the amount of light emitted between 0 and 10 degrees.Accordingly, if the feature surface is sloped at 30 degrees, the escapezone can be expected to allow light emitted at angles between −10degrees and +70 degrees that strike the surface to exit the escapesurface, whereas light emitted above 70 degrees will not be within theescape zone, and will be internally reflected. Since, in thistwo-dimensional example, the amount of light emitted between 0 and −10degrees is greater than the amount of light emitted between 70 and 80degrees, more emitted light will strike the feature surface within thecritical angle and be able to exit the escape surface.

Contrarily, the amount of light emitted between 10 and 20 degrees doesnot appear to be greater than the amount of light emitted between 60 and70 degrees. Accordingly, setting the feature slope at 20 degrees, sothat the escape zone includes the light emitted between 10 and 20degrees, and excludes the light emitted between 60 and 70 degrees, willnot increase the total amount of light that will be within the escapezone of the sloped surface, and, in this two-dimensional example, willreduce the amount of light that exits the surface, compared to a featureslope at 30 degrees. As noted above, the actual amount of emitted lightthat will be included within the escape zone will be dependent upon thethree-dimensional orientation of the feature slope relative to the givenemission pattern. The above two-dimensional example is used merely toillustrate that a maximum amount of light may be included within anescape zone that is not aligned with the particular angle of maximumemissions, particularly if the emission pattern is not symmetric aboutthis angle of maximum emissions.

One of skill in the art will recognize that a combination of differentlysloped surfaces may be provided to optimize the amount of light thatwill be included within the escape zones of these surfaces. For example,with the example emission pattern 120 and an example critical angle of40 degrees, a significant portion of the emitted light is emitted atangles less than 40 degrees. Accordingly, the escape cone of a surfacethat is parallel to the emitting surface will include all of thisemitted light at less than 40 degrees, and thus some of the exit surfacecan be left ‘unfeatured’ (flat) to enable this light to escape. To allowlight at greater than 40 degrees to exit the surface, features may beprovided with slopes at, for example, 50 degrees, so that the escapezone at these sloped surfaces will encompass light that is emittedwithin 10 and 90 degrees.

In like manner, if the emission pattern exhibits multiple peaks/lobes, acombination of sloped surfaces may be provided corresponding to theselobes to substantially increase the amount of light that is able to exitthe escape surface, as illustrated in the example profile of FIGS. 3Band 3C. As in the single feature surface example above, the slopes ofthe multiple surfaces may be selected to optimize the total amount oflight that can be expected to strike the surface features within theircritical angles. However, in this determination, it should be noted thatareas of the surface that are sloped for one lobe may adversely affectthe light emitted within the other lobe.

In some embodiments, the proportion of surface area with a given slopemay be proportional to the power associated with the lobe correspondingto that slope. For example, if a main lobe provides 80% of the totalpower, and another lobe provides 20% of the total power, 80% of theescape surface area may include features with a slope designed tomaximize the extraction of light from the main lobe, and 20% of theescape surface may include features with a slope designed to maximizethe extraction of light from the other lobe.

Heuristic techniques may be used to determine a set of slopes that arelikely to be efficient for a given emission field pattern, generallybased on a preference for fewer and slighter slopes. Alternatively,conventional modeling techniques may be applied to determine an optimalnumber and slopes of these surface features, based on the expected,predicted, or known emission field pattern associated with the lightemitting device.

The features of FIGS. 3A-3C may be provided using any of a variety oftechniques. The profiles of FIGS. 3A-3C may represent, for example, theprofile of a plurality of concentric grooves viewed along a diagonalcross section. In like manner, the profiles of FIGS. 3A-3C may representthe profile of an array of cones formed on the escape surface. One ofskill in the art will recognize that the profiles of FIGS. 3A-3C mayrepresent other shapes as well.

The size of the features may vary in width and height. In someembodiments, the features are between 1 um and 10 um in width andbetween 1 um and 10 um in height.

FIGS. 4A, 4B illustrate a top view and profile view, respectively, of aportion of an example escape surface 400 that includes features 410. Inthis example, the features 410 are an array of conic pits arranged in aperiodic, or pseudo-period pattern. The predominant slope of the walls412 of the features 410 is created based on the emission field patternof the light emitting device, as detailed above. The pits may be formedby a patterned etching of the surface 400, and the predominant slope ofthe walls 412 may be controlled by the particular etchingcharacteristics of the etchant that is used. For example, a fast-actingetchant may create a steeper slope than a slower-acting etchant. In likemanner, the slope may vary based on the diameter of the feature 410, aswell as the environmental conditions during the etching process.

The pattern of features 410 may vary. In some embodiments, the averagecenter-to-center spacing (‘pitch’) is less than twice the diameter ofthe feature 410, however, the desired ratio of unfeatured (flat) areasto featured areas of the surface may lead to a larger pitch.

One of skill in the art will recognize that the features 410 could aswell be conic structures, rather than conic pits.

FIGS. 5A, 5 B illustrate a top view and profile view, respectively, of aportion of an example escape surface 500 that includes features 510. Inthis example, the features 510 are an array of pyramids, with slopedsurfaces A, B, C. As detailed above, the slope of the surfaces A, B, Cmay be determined based on the emission field pattern of the lightemitting device, preferably to reduce the amount of light that will betotally internally reflected at these surfaces. As in the example ofFIGS. 4A-4B, patterned etching may be used to create these features 510.

Depending upon the size of the features 510, a milling process or laseretching process may also be used to create these features 510. Forexample, a series of controlled-depth laser slices that are angled alongthe planes of each of the surfaces A, B, and C will create theillustrated pyramid features 510. In like manner, a V-shaped bit may beused to mill the pairs of surfaces A-A, B-B, C-C, of the features 510,the slope of the V-shaped bit being based on the emission field patternof the light emitting device.

Although the illustrated example features 510 are equilateral pyramids,one of skill in the art will recognize that irregular shaped pyramidsmay be formed, as well. Such irregular shapes may be used, for example,to provide multiple slopes to accommodate a multi-lobe emission fieldpattern.

In some embodiments, particularly those with relatively steep slopes,some of the light that exits the sloped surfaces may strike an adjacentsloped surface, and re-enter the light emitting device, as illustratedin FIGS. 6A-6C.

In FIG. 6A, for example, most of the light that exits the slopedsurfaces will continue to travel away from the light emitting device, asillustrated by light paths 610, 611, 612, 613. Some of the light thatexits the sloped surfaces, however, may strike an adjacent surface, andre-enter the light emitting device, as illustrated by light paths 620,621. Depending upon how the light strikes the adjacent surface, it maystrike the next sloped surface at an angle within the critical angle,and exit that sloped surface, as illustrated by light path 620. However,the light may again re-enter the light emitting device, and some of thelight may be directed back toward the light emitting layer, and mayeventually be absorbed, as illustrated by light path 621.

The likelihood of the light re-entering the light emitting device willbe dependent upon the particular arrangement of features. The featuresillustrated in FIGS. 6B and 6C, for example, will exhibit a lowerlikelihood of light re-entering the light emitting device, because ofthe larger separation between the steeper slopes, although some re-entrymay occur.

In some embodiments of this invention, the surface features are formedto reduce both total internal reflection and reentry of light that exitsthe surface. As illustrated in FIG. 4A, if the sloped surfaces of thefeatures face each other in close proximity, the likelihood of lightre-entering an adjacent feature may be significant. Although some of there-entering light may exit the adjacent feature, at least some of thisre-entering light is likely to be absorbed within the light emittingdevice.

FIG. 7 illustrates an example alternative arrangement of the features510 of FIG. 5A. In this example, some of the features 510 of FIG. 5A arenot formed, thereby providing space 715 between the formed features 710of FIG. 7. This additional spacing reduces the likelihood of lightre-entering the neighboring features 710, because only veryshallow-angled light will be below the height of the neighboring feature710 after traveling this longer distance.

Additionally, in the example of FIG. 7, two similar surfaces do notdirectly face each other. In the example of FIG. 5A, each surface Afaced a similar surface A; each surface B faced a similar surface B; andeach surface C faced a similar surface C. For the purposes of thisdisclosure, two surfaces are said to be directly facing each other if anormal from each of the surfaces lie in a common plane, or lie within anangle of ten percent of a common plane.

In the example of FIG. 7, the surface areas that are in the path oflight emitted from surface A include only the sloped surfaces B and C oftwo neighboring features 710. A normal from surface A will not liewithin ten percent of either plane of a normal to surface B or C. Lightfrom surface A that strikes surface B or C will re-enter the neighboringfeature, but less light from surface A is likely to strike surface B orC, because these sloped surfaces B and C form a “valley” (or void)through which light emitted from surface A may travel unimpeded. Asimilar analysis with respect to each of surfaces B and C will showsimilar valleys for unimpeded light propagation from these surfaces.

A potential drawback of the arrangement of features 710 on the surface700, however, may be the areas 715 of the surface 700 that remain flat.If the emission field pattern includes a lobe at or near normal to thelight emitting surface, these flat spaces 715 will facilitate the exitof light of that lobe, but if the emission field pattern indicates thatvery little light is emitted normal to the light emitting surface, suchas pattern 140 of FIG. 1, these flat spaces will increase the likelihoodof light from the light emitting device being totally internallyreflected. This total internal reflection may be reduced by rougheningthese flat spaces 715, although such roughening may introduce somere-injection of light as mentioned above.

One technique for reducing the amount of non-sloped surfaces is toinclude additional features 810 within the spaces 715 between the majorfeatures 710, as illustrated in the example of FIG. 8. In this example,each surface A, B, C of the features 710 faces the edge of the surfacepairs B-C, C-A, and A-B of the features 810, respectively. Although theadditional features 810 interfere with the aforementioned “valleys”produced by the sloped surfaces B and C for light emitted from surfaceA, the reduced height of these smaller features 810 decreases thelikelihood that light emitted from surface A of the features 710 willstrike the surfaces of the features 810.

In some embodiments, the major 710 and minor 810 features may beinverses of each other. That is, the major features 710 may be pyramidsthat extend up from the surface, while the minor features 810 may bepits that extend down below the surface, or vice versa, as illustratedin FIG. 3B. In like manner, any of the particular features in any of thefigures may be pits in lieu of pyramids; for example, every other row offeatures may be an inverse of the adjacent row. These and othervariations will be evident to one of skill in the art in view of thisdisclosure.

FIG. 9 illustrates an example alternative arrangement of features 910that reduces the amount of unsloped surface areas, for emission fieldpatterns that may not have a significant lobe at or near normal to thelight emitting surface. As in the example arrangement of features 710,the features 910 are arranged such that no two surfaces directly faceeach other.

FIG. 10 illustrates an addition of other features 950 with slopedsurfaces D, E, F, that may be the same or different from the slope ofsurfaces A, B and C. These additional features further increase thenon-sloped areas on the escape surface. In this example embodiment, ifthe features 950 are designed for accommodating a second lobe in theemission field pattern, and the slopes of the surfaces D, E, F arerelatively slight, such as under 30 degrees, the likelihood of lightre-entering the light emitting device through features 910, 950 may alsobe relatively slight, even though the surface A directly faces (smaller)surfaces D, surface B faces surfaces F, and surface C faces surfaces E.Even if the surfaces D, E, F are at the same slope as the surfaces A, B,C, the ‘valleys’ produced by the sloped surfaces and the reduced heightof the features 950 substantially reduce the likelihood of emitted lightre-entering the features 910, 950, as compared to the examplearrangement illustrated in FIGS. 5A-5B.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments.

For example, although pyramid shapes are used in these examples, othershapes may be used as well. FIG. 11 illustrates an example surfacecomprising a pseudo random arrangement of a variety of surfaces atdifferent angles, suitable, for example, for a light emitting devicehaving a multi-lobe emission field pattern.

In like manner, FIG. 12 illustrates an example surface comprising aplurality of features arranged in a circular-like pattern. This examplealso illustrates the use of different sized features.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims. For example, the use of sloped surfaces for improvinglight extraction efficiency may be combined with other techniques forimproving light extraction efficiency, including, for example,roughening some or all of the surface features.

As noted above, the preferred slopes of the features on the escapesurface may be determined based on a given angular emission fieldpattern, or a preferred angular emission field pattern may be determinedbased on the slopes of the features on the escape surface, or both theslope of the features and the angular emission field pattern may bedetermined in unison with each other to optimize the light extractionefficiency based on the combination of a select angular emission fieldpattern and select slopes of the features on the escape surface. Forease of reference, the phrase “slopes corresponding to the emissionfield pattern” is intended to be independent of any particular order ofdetermining the slopes or the emission field pattern.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasures cannot be used to advantage. Any reference signs in the claimsshould not be construed as limiting the scope.

The invention claimed is:
 1. A light emitting element, comprising: afirst region of a first dopant type; a second region of a second dopanttype different from the first dopant type; a light emitting layersandwiched between the first region and the second region; an escapesurface formed in the first region; and a mirror formed at a distancefrom the light emitting layer, the mirror being configured to reflectlight from the light emitting layer toward the escape surface, wherein:each of the first and the second regions comprising one or more layers;the light emitting layer and the mirror have a relative distance of 0.3to 1 so that: the light emitting element is configured to emit a lightwith an angular emission field pattern relative to a surface of thelight emitting layer; and the angular emission field pattern comprisesat least one lobe at a peak emission angle that is between about 20 and75 degrees relative to the surface of the light emitting layer; whereinthe relative distance is defined as d=η*D/λ, η is the index ofrefraction of the material between the active layer and the mirror,distance D is a distance between the light emitting layer and themirror, and λ is the wavelength of the emitted light; the escape surfacedefines conic features; and each conic feature comprises a predominantslope that is substantially orthogonal to the peak emission angle. 2.The light emitting element of claim 1, wherein the predominant slope isbiased from the peak emission angle toward angles of greater emission.3. The light emitting element of claim 2, wherein the predominant slopeis biased to maximize an amount of light that strikes the predominantslope within its escape zone.
 4. The light emitting element of claim 1,wherein the conic features comprise conic pits or conic structures. 5.The light emitting element of claim 4, wherein the conic features arearranged in an array with a periodic or pseudo-periodic pattern.
 6. Thelight emitting element of claim 5, wherein a proportion of the escapesurface with the predominant slope is proportional to a proportion of atotal power associated with the lobe.